Like mitochondria, the chloroplasts are also double membrane bound. Of the two, the inner chloroplast membrane is relatively less permeable. The space limited by the inner membrane of the chloroplast is called the stroma. A number of organised flattened membranous sacs called the thylakoids, are present in the stroma. Thylakoids are arranged in stacks like the piles of coins called grana (singular: granum) or the intergranal thylakoids. In addition, there are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana.

On the inner wall of thylakoids are found small granules called quantasomes; their name was coined by Park and Pon (1963). Each quantasome has 230-250 chlorophyll molecules. Quantasomes were considered as photosynthetic units; however it has now been doubted. The thylakoid membranes are the site of light reaction of photosynthesis, while the stroma is the site of carbon dioxide fixation reaction of photosynthesis.

NCERT Biology Text, Class: XI, Unit: Cell: Structure and Functions, Chapter: Cell: The Unit of Life, Page no: 136 & Fundamentals of Botany By Soni, Page no: 216.